This invention relates to improvements in motion detectors used in intruder monitors, lighting control systems, robotics and other areas where presence of a moving person within boundaries of a space under surveillance is desirable. The proposed detector operates on the monitoring of abnormal rate of change in ambient temperature of such space, as would be occasioned by the movement therethrough of any object whose body heat differs from the steady state ambient temperature.
Any object emits radiation whose spectral characteristics depend on the object's temperature. If the temperature of the object is different than that of ambient, thermal energy transfer occurs between the object and the surrounding space. Humans, as objects whose movements should be detected, emit electromagnetic radiation in the wavelength range of 9 .mu.m. On the other hand, any infrared motion detector contains a sensor, whose radiated wavelength also depends on its temperature. According to Stefan-Boltzmann law, net power, Nr, transferred by radiation is governed by the following equation: EQU Nr=.epsilon..sub.o .multidot..epsilon..sub.s .multidot..sigma..multidot.A.multidot.(T.sub.s.sup.4 -.beta..multidot.T.sub.o.sup.4 -.alpha..multidot.T.sub.a.sup.4),(1)
where .epsilon..sub.o and .epsilon..sub.s are emissivities of the object and the sensor respectively (in most cases they are constant and close to unity), .sigma. is Stefan-Boltzmann constant, A is sensor's area, Ta, To and Ts are absolute temperatures of the surroundings, the object and the sensor respectively, .alpha. and .beta. are coupling coefficients between the sensor and surroundings and between the sensor and the object respectively, so that .alpha.+.beta.=1. The above equation is the scientific basis for all passive infrared (PIR) motion detectors, known in the prior art.
A PIR detector passively detects thermal radiation absorbed by the sensor. As a result, speed of response of the passive sensor greatly depends on its volume, specific heat and mounting of the sensing material.
Several types of PIR detectors are known in the prior art. Mortensen (U.S. Pat. No. 4,052,716) discloses a system with plurality of thermistors which collect radiation by means of a parabolic focusing mirror. It uses a capacitive coupling to distinguish between slow and fast moving objects. A similar detector utilizing a thermopile sensor is disclosed by Schwartz (U.S. Pat. No. 3,760,399 and Re. 29,082). In this invention, two fixed thresholds and OR gate are used to detect positive and negative going signals from the preamplifier. The location and direction of the moving object in the prior art is determined by inventions of the above cited authors and by Keller (U.S. Pat. No. 4,052,616) and Schwartz (U.S. Pat. No. 3,958,118).
Many commercially available PIR detectors use pyroelectric crystals as thermal radiation sensors. Triglycine sulfate (TGS), lithium tantalate (LiTaO3) and ceramic pyroelectrics are the most popular materials for the manufacturing of PIR detectors.
Instead of a small size solid-state sensor, a pyroelectric detector can be designed with a polymer film, such as polyvinylidene fluoride (PVDF). Cohen (U.S. Pat. No. 3,809,920) discloses a design which contains the polymer film with conductive electrodes on both surfaces. The heat flow from non-moving objects can be separated by the cancelling technique, as it was disclosed in the U.S. Pat. No. 3,839,640 or by the use of a differential amplifier with common mode rejection of noise (Smith et al. U.S. Pat. No. 4,379,971). Smith's detector utilizes a polymer film with the interdigitized (alternating) electrodes on one side and a parabolic mirror as a focusing system. The patent of Southgate (U.S. Pat. No. 3,842,276) discloses the use of an alternative electrode arrangement on the opposite surfaces of the pyroelectric film to produce ambient temperature compensation. U.S. Pat. No. 4,225,786 issued to Perlman discloses design of a passive infrared detection system with a pyroelectric detector array.
All detectors from the above inventions use either thermistors, thermocouples or pyroelectrics as discrete sensors whose temperature passively follows that of ambient. A moving object radiates thermal energy toward the sensor whose temperature deviates from that of ambient in accordance with the movement of an object. To assure an an acceptable speed response, all passive sensors, known in the prior art, must be small. This imposes substantial demands on the focusing system, especially if wide field of view is required, and increases cost of the detector.
The focusing systems in the prior art are made of lenses and reflectors to concentrate radiation on the individual sensors. Some detectors use sectional Fresnel lenses (facets) which focus radiation on a single pyroelectric or thermopile sensor (RCA Motion Switch C-23, patent pending; Visonic Ltd. Motion Sensor SR-2000E; Kesser Electronics International, Inc. Infrared Sensor, Model 2006 and others presently commercially available). Wide field of view was achieved by the use of cylindrical shape of both the sensor and the lens, as in FIRM-287 motion detector, manufactured by the American IRIS Corporation. In the later detector, the sensor and the lens are curved with the radius equal to one half of focal length of the lens. U.S. Pat. No. 4,321,594 issued to Galvin teaches application of curved and multiple facet lenses to passive infrared detectors. U.S. Pat. No. 4,450,351 issued to J. Fraden teaches application of image distortion means to passive detectors operating in visible range of spectrum and the use of passive photoresistor having snake-like shape.